FIG. 1 is a sectional view exemplifying a conventional surface illuminant device using the light transmitter. In this drawing, the reference numeral 1 represents a rectangular plate-like light transmitter having adequate thickness and a proper area; 2 is a light diffusing sheet mounted on a surface of emergence 1a of the light transmitter 1 in a condition where one end of the light diffusing sheet 2 is fixed, for example, with a double-adhesive tape 3 and having an area substantially the same as that of the light transmitter 1; and 4 is a reflecting sheet mounted on a surface 1b of the light transmitter 1 which is opposite to the surface of emergence 1a in a condition where one end of the reflecting sheet 4 is fixed, for example, with a double-adhesive tape 5 and having an area substantially the same as that of the light transmitter 1. The reference numeral 6 designates a holder which has collars 6a and 6b sandwiching opposite ends of the light transmitter 1, the light diffusing sheet 2 and the reflecting sheet 4, extends along an end surface of incidence 1c of the light transmitter 1 so as to cover the ends of the light transmitter 1, the light diffusing sheet 2 and the reflecting sheet 4 and has a substantially toric sectional shape; and 7 is a slender light source such as a cold cathode ray tube which is fixed to the holder 6 by an adequate means and extends along the surface of incidence 1c of the light transmitter 1. The reference numeral 8 denotes a bolt inserted through holes which are coaxially bored through the collar 6a, the light diffusing sheet 2, the light transmitter 1, the reflecting sheet 4 and the collar 6b; and 9 is a nut which is screwed around an end of the bolt 8 protruding from a bottom surface of the collar 6b.
In the surface illuminant device which is configured as described above, rays emitted from the light source 7 travel toward the end surface of incidence 1c directly from the light source 7 or after being reflected by a reflecting surface formed as an inside surface of the holder 6, enter the light transmitter 1 from the end surface of incidence 1c and propagate through the light transmitter 1. In the light transmitter 1, the incident rays are reflected by the reflecting sheet 4 or inside surfaces of the light transmitter 1, thereafter emerging from the surface of emergence 1a. Since the rays are diffused by the light diffusing sheet 2 while transmitting therethrough, this surface of the light diffusing sheet 2 can be used as a surface light source.
Such a surface illuminant device may ordinarily be used in environments kept at high temperature or high humidity. When the conventional surface illuminant device is used in such environments, the light transmitter 1, the light diffusing sheet 2 and the reflecting sheet 4 which have coefficients of linear expansion different from one another are expanded or contracted at ratios different from one another, whereby the light diffusing sheet 2 and/or the reflecting sheet 4 may be slackened. In such a case, the rays emerging from the surface of emergence 1a of the light transmitter 1 are influenced so as to constitute a cause for an ununiform luminance distribution on the surface illuminant device.
For solving this problem, it has conventionally been practised to dispose the light diffusing sheet 2 and the reflecting sheet 4 in a condition where opposite ends of the light diffusing sheet 2 and the reflecting sheet 4 are kept free or not fixed. When the light diffusing sheet 2 and the reflecting sheet 4 are disposed in such a condition, however, the light diffusing sheet 2 and/or the reflecting sheet 4 may tear off the surface of emergence 1a of the light transmitter 1 under certain kinds of circumstances, thereby allowing dust or foreign matters to enter a gap and/or gaps formed between the light diffusing sheet 2 and the surface of emergence 1a of the light transmitter 1 and/or between the surface of emergence 1a of the light transmitter 1 and the reflecting sheet 4 before the surface illuminant device is built into a casing. The dust or the foreign matters intercept the rays which are to emerge from the surface of emergence 1a of the light transmitter 1, thereby lowering luminance on the surface illuminant device.
In view of the problem described above, it is a primary object of the present invention to provide a surface illuminant device which is configured so as to prevent the light diffusing sheet and the reflecting sheet from tearing off the light transmitter, and allows no dust or foreign matter to enter between the light transmitter and either of these sheets without fixing an end thereof.
Another object of the present invention is to provide a surface illuminant device which is composed of parts in a number smaller than that of the parts required for composing the conventional surface illuminant device and can be assembled more easily than the conventional surface illuminant device.